Even in school in the course of botany (grade 6), the structure of the seed was a fairly simple and memorable topic. In fact, this generative organ of the plant arose as a result of a long evolutionary process and has a complex and unique structure. In our article, we will consider the features of its structural parts, the structure of a dicotyledonous seed, and also determine the biological role of plant seeds.
The appearance of the seed in the process of evolution
Plants were not always able to form seeds. It is known that life arose in water, and algae were the first plants. They had a primitive structure and reproduced vegetatively - by parts of the thallus and with the help of specialized mobile cells - zoospores. Rhinophytes were the first to land on land. They, like their future successors - higher spore plants, reproduced with the help of spores. But water was necessary for the development of these specialized cells. Therefore, when environmental conditions changed, their numbers also decreased.
The next evolutionary step was the appearance of the seed. This was a huge step forward for the adaptation and spread of many speciesplants. The external and internal structure of the seed determines the reliable protection of the embryo, surrounded by a supply of water and nutrients. This means that they increase the viability and species diversity of the planet's flora.
Seed formation process
Let's consider this process on the example of a group of plants, which is dominant in the modern world. These are representatives of the Angiosperms department. All of them form a flower - the most important generative organ. In its pistil is the egg, and the anthers of the stamens contain sperm. After the pollination process, i.e. the transfer of pollen from the anther of the stamens to the stigma of the pistil, the spermatozoa move along the germ tube to the stamen ovary, where the process of gamete fusion occurs - fertilization. As a result, an embryo is formed. When the second sperm fuses with the central germ cell, a reserve nutrient is formed. It is also called the endosperm. The structure of the seed is completed by a strong outer shell. Such a structure is the basis for the development of the future plant organism.
External structure of seeds
As already mentioned, the outside of the seed is covered with a peel. It is dense enough to protect the embryo inside from mechanical damage, temperature changes and the penetration of harmful microorganisms. But the color of the seeds varies widely: from black to bright red. This structure of the seed is easy to explain. In some plants, the color serves as a camouflage. For example, so that birds cannot see them in the soil after planting. Other plants, on the other hand,adapted to seed dispersal by various animals. Together with undigested food residues, they excrete them far beyond the parent plant's habitat.
Internal structure of a seed
The main part of any seed is the germ. This is the future organism. Therefore, it consists of the same parts as an adult plant. These are the germinal root, stalk, leaf and bud. The structure of the seed of different plants can vary significantly. In most of them, reserve nutrients accumulate in the endosperm. This is a shell that surrounds the embryo around, protecting and nourishing it throughout the entire period of individual development. But there are cases when, during the process of maturation and germination of the seed, it completely consumes the substances of the endosperm. Then they accumulate mainly in the fleshy parts of the embryo. They are called cotyledons. Such a structure is typical, for example, for pumpkins or beans. But in the shepherd's purse, the supply of substances is concentrated in the tissue of the embryonic root. The seeds of various systematic groups of plants also differ.
Features of seeds of Gymnosperms
The external and internal structure of the seed of this group of organisms is characterized by the fact that the process of formation and development of the embryo occurs on the surface of the seed coat. In addition to the main parts, the seeds of gymnosperms have a pterygoid membranous outgrowth. It helps spread the seeds of these plants with the help of wind.
Moreone feature of gymnosperm seeds is the duration of their formation. For them to become viable, it should take from four months to three years. The process of seed maturation takes place in cones. It's not fruit at all. They are specialized modifications of the escape. Some coniferous seeds can be stored in cones for decades. All this time they retain their viability. In order for the seeds to fall into the ground, the scales of the cone open on their own. They are picked up by the wind, sometimes carrying them over considerable distances. If the cones are soft, outwardly resembling nuts, they do not open themselves, but with the help of birds. Especially like to feast on seeds, various types of jays. This also contributes to the resettlement of representatives of the Gymnosperms department.
The very name of this systematic unit indicates that the embryo of the future plant is poorly protected. Indeed, the presence of endosperm guarantees only the development of the seed. But the cones of many plants open during adverse developmental conditions. Once on the surface of the soil, the seeds are exposed to low temperatures and lack of moisture, so not all of them germinate and give rise to a new plant.
Features of Flowering Plant Seeds
Compared to gymnosperms, representatives of the Flowering department have a number of significant advantages. The formation of their seeds occurs in the ovary of flowers. This is the most expanded part of the pistil and gives rise to the fruit. As a result, the seeds develop inside them. They are circled with three layers of pericarp, which differ in their properties andfunctions. Consider their structure using the example of a plum drupe. The outer leathery layer protects against mechanical damage, ensuring integrity. Medium is juicy and meaty. It nourishes and provides the embryo with the necessary moisture. The inner ossified layer is an additional protection. As a result, the seeds have all the necessary conditions for development and germination, even under adverse circumstances.
Monocot Seeds
The structure of a monocot seed is very easy to determine. Their embryo consists of only one cotyledon. These parts are also called germ layers. All plants of the Cereal, Onion and Lily families are monocots. If you germinate the seeds of corn or wheat, soon one leaflet will form from each grain on the surface of the soil. This is the cotyledons. Have you tried splitting a grain of rice into several pieces? Naturally, this is impossible. This is because its embryo is formed by a single cotyledon.
Dicot Seeds
Seeds of the Rosaceae, Solanaceae, Asteraceae, Legumes, Cabbage and many other families are somewhat different in structure. Even based on the name, it is easy to guess that their embryo consists of two cotyledons. This is the main systematic feature. The structure of the seeds of dicotyledonous plants is easy to see with the naked eye. For example, a sunflower seed is easily divided into two equal parts. This is the cotyledon of its embryo. The structure of the dicotyledonous seed can also be seen from young seedlings. Try to germinate the seeds of common beans at home. And you will see two carpels appear above the ground.
Conditions for seed germination
The structure of the seeds of dicotyledonous plants, as well as representatives of other systematic units of this kingdom of wildlife, determines the presence of all the necessary substances for the development of the embryo. But other conditions are necessary for germination. For each plant, they are completely different. First, it is a certain air temperature. For heat-loving plants, this is +10 degrees Celsius. But winter wheat begins to develop already at + 1. Water is also needed. Thanks to it, the grain swells, which speeds up the processes of respiration and metabolism. Nutrients are converted into a form in which they can be absorbed by the fetus. The presence of air and sufficient sunlight are two more conditions for the germination of the seed and the development of the whole plant, since photosynthesis is impossible without them.
Seeds and fruits
Each fruit contains seeds. The structure of the seeds of higher plants is almost identical. But the fruits are more diverse. Allocate dry and juicy fruits. They differ in the structure of the layers that are located around the seed. In succulent, one of the layers of the pericarp is necessarily fleshy. Plum, peach, apple, raspberry, strawberry… These delicacies are loved by everyone precisely because they are juicy and sweet. In dry fruits, the pericarp is leathery or ossified. Its layers usually coalesce into one,reliably protecting the seeds inside. A box of poppies, a pod of mustard, a grain of wheat have just such a structure.
The biological role of seeds
Most of the plants on the planet use seeds for reproduction. The structure of the seeds of modern plants is the result of a long evolution. These generative organs contain the embryo and a supply of substances that ensures its growth and development even under adverse conditions. Seeds have adaptations for dispersal, which increases their chance of survival and settling.
So the seed is the result of the fertilization process. It is a structure consisting of an embryo, reserve substances and a protective peel. All its elements perform certain functions, thanks to which the group of seed plants has taken a dominant position on the planet.
